According to Edward Kravitz of Harvard University, aggression is a nearly universal feature of social animals. Even flies and lobsters fight.

It turns out that male and female flies fight differently - and it all comes down to a single gene. If a fly has it, it fights like a boy, if not it fights like a girl (regardless of what sex it actually was).

Check out some fight sequences from flies and lobsters (set to music!) at the Kravitz' lab website: Link - via New Scientist Blogs